Electrochemical Reduction of Carbon Dioxide to Formate

Electrochemical CO2 reduction (CO2RR) is an environmentally friendly approach to transform greenhouse CO2 to value-added chemical feedstocks and fuels. One of the promising CO2RR products is formate which is widely applied in chemical, food and energy related industrials. The ideal CO2RR to formate electrolysers should possess features such as high formate conversion Faradaic efficiencies (FEformate) at low overpotentials, high current densities, and outstanding stability to meet industrial requirements. In this thesis, highly selective formate producing catalysts were designed and prepared. The effects of CO2RR catalysts’ structures, the electrolyte alkalinity, the cell configuration, and the full-cell assembly in combination with an oxygen evolution anode toward CO2RR performance were systematically studied. To study catalyst structural effects on formate selectivity, a novel hierarchical structure of 3 dimensional (3D) mesoporous Pd on highly ordered TiO2 nanotubes were prepared via the electrodeposition method. The product selectivity was found to depend on the TiO2 nanotube length, resulting from the influence of mass transports of CO2, protons and products in the tubes. This work demonstrates the importance of designing efficient hierarchical structures to optimise reactant/product mass transport and electrochemical kinetics. The electrochemical flow cell was employed to overcome the low current density and mass transfer challenge encountered in H-cell using SnS nanosheet-based catalysts. Alkaline electrolyte (1.0 M KOH) successfully suppressed the hydrogen evolution across all potentials particularly at the less negative potentials, and CO evolution at more negative potentials. This in turn widened the electrochemical potential window for formate conversion. A comparative study to SnOx counterpart indicated sulfur also acts to suppress hydrogen evolution, although electrolyte alkalinity resulting in a greater suppression. Moreover, to achieve a long-term current stability, it is necessary to buffer the carbonate/bicarbonate formed from chemical reactions between CO2 and KOH. High performance oxygen evolution reaction (OER) catalyst is required to be coupled with CO2RR cathode for the full-cell electrolyser assembly. The ultrathin amorphous iron oxyhydroxide nanosheets were synthesized via cyclic voltammetry (CV) potential modulations on thermally treated iron foils. The size and thickness of nanosheets were controlled by tuning CV cycles, potential range, duration, and electrolytes. By loading of Ni species onto the nanosheets, the OER activity was significantly enhanced, indicating iron oxyhydroxide nanosheets can act as excellent 2D supports to achieve synergies effect of bimetallic catalysis. A single full-cell CO2 electrolyser under electrochemical flow configuration was developed by employed CO2RR active Bi nanoparticles (NPs)-based cathode and earth-abundant NiFe layered double hydroxide (LDH) anode. The rate determining step of CO2RR to formate is the formation of *OCHO via one electron transfer. The combination of highly active NiFe LDH anode, highly efficient Bi NPs cathode, and highly conductive KOH electrolyte operated in flow cell configuration, all contribute to high-performance non-precious metal catalyst-based electrolyser. This thesis successfully developed several formate producing CO2RR catalysts, and systematically studied effects of mass transports, electrolyte alkalinity, cell configuration, and anode activity for CO2RR to formate. Such studies in catalyst development and understanding the factors influencing CO2RR performance would assist in developing commercial-relevant large-scale electrolysers

Facile electrochemical synthesis of ultrathin iron oxyhydroxide nanosheets for the oxygen evolution reaction

We propose a facile approach to synthesise ultrathin iron oxyhydroxide nanosheets for use in catalysing the electrochemical oxygen evolution reaction. This two dimensional material lowers the overpotential and provides a platform for further performance enhancement via integration of species such as nickel into an ultrathin nanosheet structure

Emerging approach in semiconductor photocatalysis: Towards 3D architectures for efficient solar fuels generation in semi-artificial photosynthetic systems

Interest in the application of semiconductors toward the photocatalytic generation of solar fuels, including hydrogen from water-splitting and hydrocarbons from the reduction of carbon dioxide, remains strong due to concerns over the continued emission of greenhouse gases as well as other environmental impacts from the use of fossil fuels. While the efficiency and durability of such systems will depend heavily on the types of the semiconductors, co-catalysts, and mediators employed, the dimensionality of the semiconductors employed can also have a significant impact. Recognizing the broad nature of this field and the many recent advances in it, this review focuses on the emerging approaches from 0-dimensional (0D)to 3-dimensional (3D)semiconductor photocatalysts towards efficient solar fuels generation. We place particular emphasis on systems that are semi-artificial , that is, hybrid systems that integrate naturally occurring enzymes or whole cells with semiconductor components that harvest light energy. The semiconductors in these systems must have suitable interfacial properties for immobilization of enzymes to be effective photocatalysts. These requirements are particularly sensitive to surface structures and morphology, making the semiconductor dimensionality a critical factor. In addition to providing an overview of advances towards designing 3D architecture in semi-artificial photosynthetic field, we also present recent advances in fabrication strategies for 3D inorganic photocatalysts

Electrochemical CO2 reduction and mineralisation in calcium containing electrolytes

One of the key challenges of room temperature aqueous CO2 electrolysis technology is the carbon losses because of carbonate formation. It is desirable if carbonate ions could be utilized concurrently for a useful process. Herein, we devise a strategy that enables in-situ electroreduction and assisted CO2 storage using a by-product of that reduction process and carbonate ions. By employing a Ag catalyst deposited on a gas diffusion layer, we demonstrate CO2 electroreduction and concurrent storage via mineralisation using seawater, as well as other calcium containing electrolytes. For example, CO2 electroreduction in 0.6 M Na2SO4 containing 400 ppm Ca electrolyte results in a Faradaic conversion efficiency to CO of ∼90 % at - 1.4 V vs. RHE (∼60 ± 6 mA cm−2), and concurrently stored CO2 as calcium carbonate. This bioinspired work offers a new avenue where CO2 storage is incorporated in a sustainable CO2 electroreduction technology

Boosting Formate Production from CO2 at High Current Densities Over a Wide Electrochemical Potential Window on a SnS Catalyst

The flow-cell design offers prospect for transition to commercial-relevant high current density CO2 electrolysis. However, it remains to understand the fundamental interplay between the catalyst, and the electrolyte in such configuration toward CO2 reduction performance. Herein, the dramatic influence of electrolyte alkalinity in widening potential window for CO2 electroreduction in a flow-cell system based on SnS nanosheets is reported. The optimized SnS catalyst operated in 1 m KOH achieves a maximum formate Faradaic efficiency of 88 ± 2% at −1.3 V vs reversible hydrogen electrode (RHE) with the current density of ≈120 mA cm−2. Alkaline electrolyte is found suppressing the hydrogen evolution across all potentials which is particularly dominant at the less negative potentials, as well as CO evolution at more negative potentials. This in turn widens the potential window for formate conversion (\u3e70% across −0.5 to −1.5 V vs RHE). A comparative study to SnOx counterpart indicates sulfur also acts to suppress hydrogen evolution, although electrolyte alkalinity resulting in a greater suppression. The boosting of the electrochemical potential window, along with high current densities in SnS derived catalytic system offers a highly attractive and promising route toward industrial-relevant electrocatalytic production of formate from CO2

Progress and perspectives for electrochemical CO2 reduction to formate

Electrochemical CO2 reduction (CO2RR) is an environmentally friendly approach to transform greenhouse gas CO2 to value-added chemical feedstocks and fuels. One of the most promising CO2RR products is formate with widespread commercial applications across chemical, food, and energy related industrials. An ideal high performing CO2 electrolyser to synthesis formate should operate stably with high formate conversion efficiencies, at high current densities and low voltage that meeting industrial technoeconomic requirements. Significant progresses have been achieved in the past decades in the development of advanced catalysts, electrolyte engineering, and electrolyser designs that improved overall CO2 electrolysis performance. In-depth fundamental understanding of electrocatalytic reaction mechanisms was achieved through advanced in-situ analytical techniques. Although lab-scale electrolysers are relatively well-developed, it is still not reaching maturity level for industrial formate manufacturing that requires stable and efficient cell performance at economic scales. Here, CO2RR mechanistic studies including the employed advanced techniques for formate production are reviewed. Recent advances in the syntheses of p-block post-transition and transition metal-based catalysts and their performances are discussed. The main strategies for performance improvements including catalyst optimisation, electrolyte control, and cell designs, are critically assessed. Finally, we offer perspectives on future developments of CO2RR to formate

Revisiting the Role of Discharge Products in Li–CO2 Batteries

Rechargeable lithium-carbon dioxide (Li–CO2) batteries are promising devices for CO2 recycling and energy storage. However, thermodynamically stable and electrically insulating discharge products (DPs) (e.g., Li2CO3) deposited at cathodes require rigorous conditions for completed decomposition, resulting in large recharge polarization and poor battery reversibility. Although progress has been achieved in cathode design and electrolyte optimization, the significance of DPs is generally underestimated. Therefore, it is necessary to revisit the role of DPs in Li–CO2 batteries to boost overall battery performance. Here, a critical and systematic review of DPs in Li–CO2 batteries is reported for the first time. Fundamentals of reactions for formation and decomposition of DPs are appraised; impacts on battery performance including overpotential, capacity, and stability are demonstrated; and the necessity of discharge product management is highlighted. Practical in situ/operando technologies are assessed to characterize reaction intermediates and the corresponding DPs for mechanism investigation. Additionally, achievable control measures to boost the decomposition of DPs are evidenced to provide battery design principles and improve the battery performance. Findings from this work will deepen the understanding of electrochemistry of Li–CO2 batteries and promote practical applications

A Self‐Assembled CO2 Reduction Electrocatalyst: Posy‐Bouquet‐Shaped Gold‐Polyaniline Core‐Shell Nanocomposite

© 2020 Wiley-VCH GmbH Here it was demonstrated that the decoration of gold (Au) with polyaniline is an effective approach in increasing its electrocatalytic reduction of CO2 to CO. The core-shell-structured gold-polyaniline (Au−PANI) nanocomposite delivered a CO2-to-CO conversion efficiency of 85 % with a high current density of 11.6 mA cm−2. The polyaniline shell facilitated CO2 adsorption, and the subsequent formation of reaction intermediates on the gold core contributed to the high efficiency observed